The hexose-6P: phosphohydrolase involved in the phosphoenolpyruvate-dependent futile cycle in Streptococcus lactis has been purified and characterized. Xylitol mediated inhibition of growth of Lactobacillus casei is due to the establishment of a PEP: pentitol futile cycle. Fourier transform, high resolution 31P-NMR has been used to study sugar transport, and in vivo mechanisms for regulation of glycolysis, in physiologically intact cells of S. lactis. Lactose analogs have been prepared enzymatically, in which (14C) 2-deoxy-D-glucose or (14C) 2-fluoro-D-glucose replace glucose as the aglycon substituent of the disaccharide. The transport of the analogs by mannose: PTS and glucokinase defective mutants has been studied, and intracellular products have been identified. Evidence has been obtained for intracellular phosphorylation of glucose analogs by the mannose: PTS in streptococci.